Paper mill core structure for improved winding and support of paper mill roll

ABSTRACT

Paper mill cores of the invention are structured to allow winding and unwinding chucks to distort the interior diameter and shape of the interior body wall of the core while resisting distortion of the core exterior to prevent center burst failure. The paper mill cores of the invention include a multi-ply zone of high strength, but relatively compliant, paperboard plies within the outer 70% (based on the total body wall thickness) of the body wall having a thickness of at least about 4 mm. An interior zone constituting at least about 25% of the total thickness of the body wall is formed from extremely high strength, extremely high density paperboard plies. The overall wall thickness of the core is preferably at least about 15 mm, and is thus preferably increased, as compared to the wall thickness of conventional high strength wide paper mill cores formed entirely of extremely high strength, extremely high density, non-compliant paperboard plies.

FIELD OF THE INVENTION

[0001] The invention is directed to a paperboard core structure for winding and supporting heavy rolls of wide, continuous paper sheet. More particularly, the invention is directed to a multilayer paperboard core structure having a high flat crush strength and dynamic strength for winding and supporting rolls of continuous paper mill sheet having a width exceeding 100 in. (254 cm.), and a roll diameter typically above about 50 in. (127 cm.).

BACKGROUND OF THE INVENTION

[0002] Wide, heavy paperboard cores for supporting wide and heavy paper mill rolls used in high end applications such as gravure printing, are constructed to meet a demanding set of strength requirements including a high flat-crush strength and a high dynamic strength. These and other strength requirements are necessary because the paperboard cores are supported internally at their ends by expanded chucks during winding and unwinding operations in which the roll of paper wound onto the core has a weight above two tons (1,800 kg), typically approaching or exceeding five tons (4,500 kg.), and spans a width typically between about 100 inches and 140 inches (254 cm. and 356 cm.). These cores are currently supplied in two standard internal diameter (ID) sizes of 3 in. (76.2 mm.) and 6 in. (152.0 mm) in the United States, and in a standard ID of 150.4 mm in Europe, (which corresponds to the 6 in. ID U.S. paper mill core).

[0003] The problem known as “center burst” failure of these heavy and wide paper rolls has frustrated the paper mill industry for years. In particular, paper rolls that appear perfect when wound, subsequently fail for no apparent reason in the unwinding operation during printing. The symptoms of center burst failure are well known and consistent. Paper in the portion of the roll near the core, and at the ends of the roll above the unwind chucks, bursts, and patches can be squeezed sideways out of the roll. These patches significantly increase the chances of a web break during the printing process resulting in costly downtime at the press room. Also, patches can lay on top of the remaining sheet causing misprints and quality problems. Such defective paper rolls returned to the paper mill supplier can have a major impact on profitability.

[0004] The exact cause of center burst failure has not been identified. For example, examination of paperboard cores supporting paper mill rolls having center burst failure defects has not resulted in identification of any correlated defects in the cores. Center burst failure occurs in about 1-8% of paper mill rolls and it is known that the frequency of center burst failure has been increasing. This increase has been attributed to the use of wider and heavier paper rolls. Also, as compared to fine quality papers of prior decades, current fine quality papers are often thinner with lower friction surfaces and often have lower strength due to a higher recycled fiber content. As a result, it is much more difficult to maintain winding tension and to build a uniform structure throughout the diameter of the paper roll. These problems have been addressed by modifications of paper mill winding apparatus to provide continuous monitoring and control of paper tension during winding along with the application of torque in such a way as to give a desired profile of winding tension. These modifications build friction and compression into the wound paper mill roll in order to minimize possible harm to the paper roll due to slipping of various layers on the roll during shipping and subsequent unwinding. Nevertheless, center burst failure persists and continues to increase with no predictable pattern or cause.

[0005] The assignee of the present application has developed various paperboard core structures and techniques to address specific problems with paperboard cores designed for specific end uses. For example, U.S. Pat. No. 5,393,582 issued Feb. 28, 1995 to Yiming Wang, Monica McCarthy, Terry D. Gerhardt, and Charles G. Johnson discloses paperboard tube structures of enhanced flat crush strength. These structures involve the use of zones or layers of paperboard plies distributed within the tube wall such that lower strength, lower density paperboard plies are positioned on the exterior and interior portions of the tube wall, while higher density, higher strength paperboard plies are positioned in the middle or central portion of the paperboard tube wall. U.S. Pat. No. 5,505,395 issued Apr. 9, 1996 to Yanping Qiu and Terry D. Gerhardt discloses paperboard tube structures that address tube inside diameter deformation problems which arise when tubes are supported on a mandrel and subjected to substantial radial compression loading as a result of highly retractive yams or films wound onto the core under high tension. These core structures involve the use of zones of high strength, high density paperboard plies positioned on the exterior and interior portions of the tube wall with lower strength, lower density paperboard plies positioned in the middle or central portion of the paperboard wall. In addition, factors influencing radial crush strength of paperboard tubes are discussed in T. D. Gerhardt, External Pressure Loading of Spiral Tape Paper Tubes: Theory and Experiment, Journal of Engineering Materials and Technology, Vol. 112, pp. 144-150 (1990).

[0006] Although these and other tube structures and modifications have been proposed for dealing with specific paperboard core end use requirements, the center burst problem associated with the wide heavy paperboard cores for wide heavy paper mill sheet rolls has not been shown to be caused by any property or apparent defect in paperboard cores. In addition, the extreme weight of paper mill paper rolls and the extreme dynamic stresses applied to the paperboard core during winding operations dictate that these paperboard cores must exhibit high flat crush and dynamic strength, thus limiting the range of core modifications available to address possible core structure variants within the desired wall thickness range to decrease center burst failure. Possible core modifications are further limited in that the inside diameter portions of the paperboard core are formed from extremely high strength, high density paperboard plies because of the so-called chuck “chew-out” forces applied by the surface of the winding chuck to the inside surface of the core during winding. Indeed, because of these various requirements, paperboard cores for wide, heavy paper mill rolls are conventionally constructed entirely of extremely high strength, high density paperboard plies. Typically, the paperboard plies have a density exceeding 0.80 g/cc and a sufficient number of plies are used in the case of a 6 inc. (150 and 152 mm) internal diameter paperboard core, to provide a wall thickness of about 13 mm (0.512 inch), or in the case of a three inch inside diameter paperboard core, a wall thickness of about 16 mm (0.630 inches).

[0007] More recently, the assignee of the present application has modified the traditional paper mill winding core constructions to incorporate the optimized flat crush strength constructions disclosed in U.S. Pat. No. 5,393,582 issued Feb. 28, 1995 to Yiming Wang, Monica McCarthy, Terry D. Gerhardt, and Charles G. Johnson. Accordingly, the current paper mill winding core constructions of assignee employ zones of paperboard plies at the exterior and interior portions of the tube wall in which the ply density is within the lower portion of the high strength range (0.80 to 0.92 g/cc), together with a central zone of plies having a density in the upper portion of the high strength range (0.80 to 0.92 g/cc). The center burst performance of these cores generally exceeds, or is at least comparable to, the center burst performance of conventional, competitive paper mill winding cores.

[0008] Nevertheless, despite the high strength and high durability of the conventional paperboard core structures, and of Assignee's modified core constructions; and even though winding apparatus has been modified to optimize the build of paper mill rolls, occurrences of center burst failure persist and have been increasing.

SUMMARY OF THE INVENTION

[0009] The invention provides paperboard core structures that can substantially reduce or eliminate center burst defects in wide, heavy paper mill rolls. The paperboard core structures of the invention are based on identification of a previously unrecognized cause of core burst failure, and on new modifications of the paperboard cores to counteract the newly identified problem.

[0010] In particular, the inventors have found that chucks used during winding operations significantly deform the exterior diameter and shape of the paperboard core. The deformation is not apparent, however, because after winding is complete and the chucks are disengaged, the core will normally return to its original size and shape. Because the diameter and shape of the core is deformed during the winding operation, the compressive and friction stresses built into the paper roll are based on the distorted shape of the core during the winding operation. However, when the chucks are disengaged after winding and the core returns to its original size and shape, a significant portion of the beneficial effects of the compressive and friction stresses built into the paper roll during winding can be lost. The distortion of the paperboard core and the paper roll build during winding, and the related changes to the paper roll structure upon removal of the core from the winding chucks, are complicated by further subsequent variable stresses that can be applied to the paper roll by the unwinding chucks inserted into the ends of the paper mill core during the unwinding operation. It is believed that the distortions of the paperboard core caused by the pressure of the unwinding chucks during the unwinding operation can, in some cases, aggravate harmful effects of the distorted stresses generated during winding and/or aggravate the loss of beneficial stresses upon removal of the core from the winding chucks, to thereby increase the possibility of core burst failure. However, in other cases, forces applied by the unwinding chucks to the core ends may counteract, at least in part, the loss of beneficial stresses and/or the distorted winding stresses built into the paper roll by the distorted core during the winding process. It is believed that the dependency of core burst failure on these separate events associated with winding and unwinding has further interfered with identification of possible causes of center burst failure.

[0011] In accordance with the present invention, paperboard cores for paper mill winding of wide and heavy paper rolls, are modified to significantly decrease or minimize outward transmission of forces applied to the inside of the core by winding and unwinding chucks. In particular, the paper mill cores of the invention are structured to allow winding and unwinding chucks to distort the interior diameter and shape of the interior body wall of the core while resisting distortion of the core exterior, i.e., the corresponding distortion of the core exterior is significantly less or substantially minimized. The modifications of paper mill cores according to the invention can be achieved while maintaining high flat crush and dynamic strength properties as are required for paper mill cores.

[0012] Paperboard core structures for counteracting center burst failure provided according to the invention include a multi-ply zone of high strength, but relatively compliant, paperboard plies within the outer 70% (based on the total body wall thickness) of the body wall. The zone of relatively compliant, high strength paperboard plies has a thickness of at least about 4 mm. In addition, the overall wall thickness of the core is preferably at least about 15 mm, and is thus preferably increased, as compared to the wall thickness of conventional high strength wide paper mill cores formed entirely of extremely high strength, extremely high density, non-compliant paperboard plies.

[0013] Advantageously the relatively compliant, high strength paperboard plies have a density of between about 0.65 and about 0.75 g/cc, more preferably, between about 0.67 and about 0.73 g/cc. In the case of the 6 in. inside diameter core (including both the 152 mm. U.S., and the 150 mm. European versions), the zone of relatively compliant, high strength paperboard plies is preferably positioned in the central portion of the paperboard wall. In the case of cores having an inside diameter of about 3 in. (76 mm.), the zone of relatively compliant, high strength paperboard plies is preferably positioned in the outer 50% of the body wall, and it is currently preferred that this zone form the exterior 40% of the body wall.

[0014] The improved core structures according to the invention substantially reduce outside diameter changes caused by winding chucks, and also decrease transfer of harmful forces to the paper roll by unwinding chucks. In accord with the invention, this is achieved by reliance on a zone of somewhat more compliant paperboard plies rather than by further strengthening the wall of the core for reasons explained in greater detail subsequently. The preferred increased core wall thickness effectively compliments the zone of more compliant, high strength paperboard plies to maintain the overall flat crush strength and dynamic strength of the paperboard core. Moreover, preferred increases in wall thickness can also increase the maximum allowable winding speed during winding and unwinding operations (known as critical speed).

[0015] In greatly preferred embodiments of the invention, the 6 in. (150 or 152 mm.) inside diameter paper mill core has a total wall thickness of above about 15 mm., more preferably about 16 mm., and thus has an increased wall thickness as compared to the wall thickness of 13 mm. conventionally used in wide, high strength 6 in. ID cores. Advantageously, the interior 25-40% of the body wall thickness is formed from extremely dense, high strength paperboard plies in which the density ranges from about 0.80 to about 0.92 g/cc. The central 30-35% of the body wall thickness is preferably formed of high strength, more compliant paperboard plies having a density between about 0.65 and 0.75 g/cc. The exterior 30-35% of the core wall is preferably formed of the extremely dense, high strength paperboard used to form the interior zone of the body wall, as discussed above.

[0016] In the case of a 3 in. (76 mm), inside diameter paperboard core according to the invention, it is preferred that the interior 55-65% of the core body wall thickness be formed from the extremely dense, high strength paperboard plies, set forth above, and that the exterior 35-45% of the wall thickness of the 3 in. (76 mm), inside diameter core be formed from the high strength, more compliant paperboard plies discussed above. The 3 in. (76 mm.), ID cores preferably have a wall thickness of about 17-19 mm as compared to the conventional 15 mm wall thickness used in 3 in. (76 mm), ID paper mill cores.

[0017] According to yet another embodiment of the invention, a portion of the high strength, more compliant paperboard plies can be mixed with the extremely high strength paperboard plies in the interior 30% the core body wall. Nevertheless, in this embodiment, sufficient extremely high strength paperboard plies are provided in the interior 30% of the core body wall such that at least half of the plies in this portion of the body wall are extremely high strength plies; and all, or substantially all of the plies forming the interior 15% of the body wall are extremely high strength plies. The total body wall thickness exceeds about 15 mm; the total thickness of plies in the lower density, high strength but compliant range exceeds about 5 mm; and the total thickness of plies in the extremely high density range exceeds about 9 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the drawings which form a portion of the original disclosure of the invention;

[0019]FIG. 1 schematically illustrates in partial perspective view a winding chuck, shown in cross section, inserted into the end of a conventional paper mill core supporting a portion of a paper roll, with the lugs of the winding chuck being shown distorting the outside diameter and shape of the paperboard core in exaggerated detail;

[0020]FIG. 2 schematically illustrates in partial perspective view an unwinding chuck (show in cross-sectional view) of different configuration as compared to the winding chuck shown in FIG. 1, inserted into the end of a paperboard core and supporting a portion of a paper roll, with the lugs of the unwinding chuck shown engaging the interior of the paperboard core in a configuration correspondingly different than the engagement of the winding chuck of FIG. 1 with the core;

[0021]FIG. 3 illustrates a perspective schematic view of a conventional paperboard paper mill core and illustrates the varying distortion of the exterior diameter and shape of the core in three different zones along the length of the core as a result of the force applied by a conventional winding apparatus;

[0022]FIG. 4 is a partially broken away cross-sectional view schematically illustrating one preferred paperboard core according to the invention;

[0023]FIG. 5 is a partial cross-sectional view illustrating interaction between the preferred core shown in FIG. 4 and the lugs of a winding or unwinding chuck;

[0024]FIG. 6 is a partially broken away cross-sectional view illustrating another preferred paper mill paperboard core according to the invention; and

[0025]FIG. 7 is a partially broken away cross-sectional view illustrating yet another advantageous paper mill paperboard core according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

[0027]FIG. 1 illustrates a conventional paper mill paper core winding process wherein a portion of a paper roll 10 is shown supported on a conventional paperboard core 12, which in turn is supported within its interior end portions by a winding chuck 14 which includes a plurality of radially protruding lugs 16. As is well known to those skilled in the art, the lugs 16 can be moved radially inwardly and radially outwardly by various well known mechanisms (not shown) in order to allow mounting of the paperboard core 12 on the chuck 14 and engagement of the paperboard core 12 by the chuck 14. In particular, when the lugs 16 are in a radially retracted position (not shown) the paperboard core can be coaxially mounted on the chuck 14. Retraction of the lugs 16 at the completion of a winding operation also allows removal of the paperboard core 12 and the full paper roll 10 from the chuck 14. On the other hand, when the lugs 16 are extended radially outwardly, the lugs forcefully contact the inside surface of the paperboard core 12 so that rotation of the chuck 14 also rotates the core 12.

[0028] The conventional chucks 14 extend axially only partially into the ends of the core 12, typically for a length of between about 2 inches (51 mm) and about 5 inches (127 mm). Because of the extremely high weight of the paper roll 10, (i.e., exceeding two tons (4000 lbs., 1,800 kg), more typically approaching or exceeding five tons (10,000 lbs., 4,500 kg.)), and also because of its substantial length, (typically between about 100 in. (2.5 m) and 142 in. (3.6 m)), and further because of the substantial torque that must be transmitted from the chuck 14 to the conventional paperboard core 12, the lugs 16 of the chuck 14 are designed to expand radially outwardly. The lugs 16 are driven outwardly by pressure from a torque activation mechanism that forces the lugs 16 outwardly a distance sufficient to engage the interior surface of the core

[0029] The conventional paperboard core 12 used for paper mill winding of wide, heavy paper rolls is formed entirely of a plurality of plies of extremely high density, extremely high strength paperboard. Typically, the conventional 6 in. ID core is formed from about 20-25 plies of extremely high density and high strength paperboard, that is, paperboard having a density typically greater than about 0.80 g/cc, ranging up to about 0.92 g/cc.

[0030] It was recognized by the present inventors that the extremely high density of the paperboard plies that are used to form the conventional core for wide, heavy paper rolls, is of such a high density that absorption of the chuck forces applied to the conventional paperboard core 12, by compression of the wall of paperboard core 12, is theoretically limited. Accordingly, the inventors designed experiments applying a plurality of miniature strain gauges to the surface of a conventional winding core in order to measure the effects of the conventional winding chuck 14, such as that illustrated in FIG. 1, to a conventional paperboard core 12 during a real time winding operation. As a result of these tests, it was found that the exterior of the paperboard cores were significantly distorted during the winding process. The distortion of the paperboard core at its ends is illustrated in FIG. 1. In particular, the lugs 16 interact with the corresponding overlying portion of the conventional paperboard core illustrated in the area 20 shown in FIG. 1 such that these portions of the core are bent outwardly causing the shape of the core to evolve towards the shape of a square (or other polygon) as illustrated in FIG. 1. Thus, because of the extremely high density of the paperboard core, distortion of the interior shape of the core by the lugs 12 produces a corresponding distortion of the exterior of the core 12. In turn, the areas 22 of the paper roll 10 that overlie the extended areas 20 of the paperboard core 12 are forced outwardly to a greater extent than the areas 24 of the roll 10 which do not overlie a lug 16 of the chuck 14.

[0031] It was also found that when the winding operation has been completed and when the fully wound roll of paper 10 supported on the conventional winding core 12, is removed from the chuck 14, the significant circumferential strains observed on the surface of the paperboard core 12 are relieved. As a practical matter, this means that the paperboard core 12 returns to its original circular shape. Thus, the areas 22 of the paper roll 10 wound on top of the extended portions 20 of the paperboard core 12 can lose the beneficial effects of radial and friction stresses developed during the winding process, that in turn can help maintain roll integrity until the unwind operation. In addition, it is believed that in some cases the areas 22 of the paper roll 10, can also retain harmful effects as a result of differential pressure, corresponding to the distorted outside shape of the paperboard core, during winding.

[0032] With reference to FIG. 2, when the roll of paper 10 is used by the end user, i.e. by the printer, the previously distorted paperboard core 12 is supported within its interior ends by an unwinding chuck 34. However, in many cases, the design and construction of the unwinding chucks 34 is completely different than the construction of the winding chucks 14. This is shown for the purposes of illustration only by the illustration of three lugs 36 in the unwinding chuck 34 as opposed to the four lugs 16 in the winding chuck 14.

[0033] During unwinding, expansion of the lugs 36 of the unwinding chuck 34 applies substantial radial force to the ID and therefore the OD of the previously distorted paperboard core 12, as indicated by force lines 38. Because of differences between the winding and unwinding chuck structures, or between the placement of the roll ends with respect to the chuck lugs during the different winding and unwinding operations, the outside of the core at the ends can transmit a substantially different pattern of forces to the paper roll during unwinding, (illustrated in FIG. 2 as triangularly distributed forces) as compared to the forces applied to the paper roll by the distorted core during winding.

[0034] In some cases, the portion of the paperboard core 20 that was distorted outwardly during the winding operation illustrated in FIG. 1, can be positioned during unwinding at a location, identified as location 40 in FIG. 2, between two extended lugs 36 and no longer overlying the extended lugs as was the case during winding, i.e., overlying lugs 16 shown in FIG. 1. Accordingly, significant radial pressure relief can occur in such a situation between the core and the paper in the roll 10 in the areas 42 overlying the previously extended portion of the paperboard core. Similarly, areas 44 of the paper roll 10 which overlie the extended lugs 36 of the unwinding chuck 34, can correspond to areas 24 of the paper roll shown in FIG. 1, that were positioned between, and not above, the extended lugs 16 of the winding chuck 14. This can result in application of strains to the paper roll that are distributed circumferentially in a significantly different pattern, as compared to winding strains, to thereby aggravate any harmful effects retained in the paper roll as a result of the distorted stresses applied during winding. On the other hand, if the lugs 36 of the unwinding chuck 34 are aligned with the paper roll in a manner that substantially corresponds to the original alignment of the lugs 16 of the winding chuck 14 during the winding operation, the stresses applied to the paper roll by the unwinding chuck 34 can potentially mitigate damage resulting from the loss of beneficial radial and friction winding stresses that occurred upon disengagement of the winding chucks (as discussed earlier).

[0035] The dynamic stresses applied to the paper roll at its ends during winding imply the presence of dynamic shear strain (axial sliding of the sheet forming the paper roll between the layers) at the ends of the roll. Moreover, because the dynamic stresses are substantially reduced after 100-200 mm of paper are outwardly wound on the paper roll, the dynamic shear strain is generally present only at the interior portions of the paper roll, i.e., the portions of the paper roll in which the paper is damaged in center burst failure of a paper roll during a printing operation.

[0036] Turning now to FIG. 3, the distorted shape of the paperboard core during winding is illustrated. As seen in FIG. 3, the wide paperboard core 10 includes three zones along its length including two end portions 50 and a longer middle portion 52. As generally illustrated in FIG. 3, the circumferential deformation of the core occurs generally in the end portions 50 of the core whereas the middle portion 52 of the core is not distorted because the chuck 14 as shown in FIG. 1 extends only into the end portions 50 of the core as illustrated in FIG. 3. In addition, as illustrated generally in FIG. 3, the portions of the core 20 that overlie an extended lug 16 of the winding chuck 14 shown in FIG. 1, are extended outwardly more than the areas 24 of the end portions 50 of the core which are located between the areas 20. Accordingly, the complexity of the compressional and frictional forces applied during winding to the portion of the paper roll near the core will be apparent from the distorted core shape generally illustrated in FIG. 3.

[0037]FIG. 4 illustrates a preferred core structure in accordance with the present invention. The core structure of the invention 100 illustrated in FIG. 4 is currently preferred structure for 6 in. (150 or 152 mm) ID paper mill cores of the invention. As illustrated in FIG. 4, the wall of the core 100 includes three multilayer zones 102, 104, and 106 positioned sequentially from the interior portion of the wall of core 100 to the exterior portion of the wall of core 100. Each of the three zones 102, 104, and 106 include a plurality of paperboard plies 102 a, 104 a, and 106 a respectively. The plies 102 a in the zone 102 are formed of extremely high density, extremely high strength paperboard, that is, paperboard having a density exceeding about 0.80 g/cc, advantageously between about 0.80 g/cc and about 0.92 g/cc, and preferably have a density of about 0.82 g/cc or higher, and most preferably a density in the range of between about 0.82 and about 0.90 g/cc.

[0038] Paperboard densities are determined for the purposes of the subject invention in accordance with the TAPPI 220 and 411 standard tests. According to these tests, the paperboard is fully conditioned at 73° plus or minus one degree F and at 50% plus or minus 2% relative humidity until it reaches equilibrium. Thereafter at least five samples of paperboard are measured for thickness and area and are weighed. Density is then determined by dividing the weight in grams by the volume in cubic centimeters.

[0039] Returning now to FIG. 4, the multiple plies 106 a forming zone or layer 106 of the wall of core 100 as shown in FIG. 4, are also advantageously formed of extremely high density, extremely high strength paperboard as discussed above in connection with zone 102 of FIG. 4. The multiple plies 104 a forming the central or middle zone 104 of the wall of paperboard core 100 are formed from a high strength but relatively compliant paperboard, i.e., paperboard having a density of between about 0.65 and about 0.75 g/cc, more preferably between about 0.67 and about 0.73 g/cc. The outermost ply or plies 110, which is optional, can be a ply or several plies (i.e., one to three plies, typically one or two plies) that are different from the extremely high density, extremely high strength paperboard plies of zone 102, and also different from the high strength but relatively compliant paperboard plies of zone 104. In this regard, is to be noted that outer plies of winding cores are often chosen in order to impart various surface friction or decorative aspects to the exterior of the core body; and/or to improve the manufacturing process, e.g., a spiral winding or linear draw process; and/or to improve adhesion of the exterior ply; as will be apparent to those of skill in the art. Similarly, the interior-most ply or plies 111 of the core body can be varied in the manner and for the reasons set forth above in regard to the ply or plies 110.

[0040] For reasons discussed below in connection with FIG. 5, zone 104 of the wall of the core illustrated in FIG. 4, has a thickness 114 preferably of at least about 4 mm. Even more preferably, zone 104 has a thickness 114 greater than about 4.5 mm, preferably greater than about 5 mm, more preferably between about 4.5 and 6.5 mm. Currently, it is preferred that zone 104 have a thickness of between about 5 and about 6 mm, most preferably about 5.6 mm. It is also preferred that the zone 104, formed of the compliant, high strength paperboard plies, constitute between about 25 and 40% of the total wall thickness of the core preferably between about 30 and 35% of the wall thickness.

[0041] The zones 102 and 106 formed of the extremely high density, extremely high strength paperboard plies, each have a thickness, 116 and 118, respectively, constituting between about 25 and about 40% of the total wall thickness of the core 100. Preferably, each of zones 102 and 106 constitutes between about 30 and 35% of the total wall thickness of the core 100. It is currently preferred that each of the zones 102 and 106 have a thickness, 116 and 118, respectively, constituting about 33% of the total wall thickness of the core 100.

[0042]FIG. 5 illustrates in exaggerated detail how the core structures of the invention interact with the radially expanding lugs 16 of a conventional chuck 14. In particular, as seen in FIG. 5, radial outward expansion of the lugs 16 causes the intermost zone 102 formed from extremely high density, extremely high strength paperboard plies to deform out a cylindrical configuration. Thus, each of the portions 120 a of zone 102 of the core body wall are pushed outwardly to form “corners or verticies” of a square-like or polygon-like shape. However, because the multiple plies 104 a of zone 104 are a relatively compliant and less dense paperboard material, the portions 120 b of the zone 104 which overlie the lugs 16, are capable of absorbing all or a substantial portion of the radial outward expansion of the portion 120 a of the interior zone 102 of the body wall of the core 100. The portion 120 c of the exterior zone 106 of the wall of the core 100 is preferably expanded radially outwardly to only a minimal extent because of the strain energy absorption by zone 104.

[0043] In general, and particularly in the construction of 6 inch interior diameter paperboard cores such as illustrated in FIGS. 4 and 5, it is preferred that a zone of extremely high strength, extremely high density paperboard, i.e., zone 106 in FIG. 4, be positioned outwardly of the relatively compliant, high strength paperboard zone 104. This is believed to enhance the absorption by the zone 104 of the radial expansion effected by the lugs 16, without causing substantial exterior distortion of the core body 100.

[0044] Because of the extremely high density of the paperboard plies forming the interior and exterior zones 102 and 106, respectively, of the body wall 100 shown in FIGS. 4 and 5, the radial outward expansion of the lug 16 of the chuck 14 (as illustrated in FIG. 5) does not significantly compress the thickness 116 of the interior zone 102, or the thickness 118 of the exterior zone 106 of the body wall. However, because zone 104 is formed of a relatively compliant, lower density paperboard, the thickness 114 of zone 104 can compress, particularly in those portions 120b of zone 104 which overlie the outwardly expanding lugs 16 of the chuck 14. In general, the total thickness and number of plies in the relatively compliant zone 104 are selected to allow absorption of the expansion distance of the chuck lugs 16. Thus, where lower density paperboard materials within the lower portion of the preferred range are selected for the formation of the zone 104, the total thickness of zone 104 can be less as compared to the situation where higher density paperboard materials within the preferred range are selected for the formation of the zone 104.

[0045] In a preferred embodiment, the total number of plies used to form the body wall of core 100 as shown in FIGS. 4 and 5, will range from about 25 to about 35 plies, preferably from about 28 to about 32 plies. In general, the higher density plies will typically have a smaller thickness as compared to the lower density plies. For example, the body wall of the core 100 can advantageously be formed of a plurality of extremely high density, extremely high strength paperboard plies each having a total thickness of about 0.022 inch (0.56 mm), and a plurality of relatively compliant, high strength paperboard plies having a thickness of about 0.025 inch (0.64 mm).

[0046] As will be well understood by those of ordinary skill in the art, the thickness and density of paperboard plies can be widely varied. Preferably, the paperboard plies used in the invention will each have a density between about 0.65 and about 0.92 g/cc, more preferably between about 0.67 and about 0.90 g/cc. Paperboard strength and density are generally varied by varying pulp treatments, by varying the degree of nip compression and by variations in the raw materials forming the pulp. Paperboard densities and strengths can also be changed by employing various known additives and strengthening agents during the papermaking processes. Paperboard plies useful herein will typically have a thickness within the range of between about 0.020 inch (0.51 mm) and about 0.035 inch (0.89 mm), more typically between about 0.022 inch (0.56 mm) and about 0.030 inch (0.76 mm).

[0047] In general, the present invention addresses a number of previously unrecognized problems and causation factors associated with the center burst problem. Significantly, the strength of extremely high density, extremely high strength paperboard plies such as are normally used to form the body wall of wide paper mill cores is derived in large part from a high pressure compression of high quality paper pulp. However, the resultant high density, high strength paperboard retains extremely little capacity for further compression, i.e., for further reduction in thickness. Accordingly, the present invention relies upon the use of a plurality of relatively strong paperboard plies, each still retaining the capacity of further compression and thickness reduction. Therefore, the lower density, high strength paperboard plies employed in the present invention are capable of absorbing a substantial amount of the radial expansion of the lugs of a conventional chuck to thereby counteract the radial expansion that would otherwise be transmitted in a corresponding amount to the exterior of the paperboard core.

[0048]FIG. 6 illustrates a preferred core structure of the invention as applied to a 3 inch (76 mm) inside diameter paper mill core. The core illustrated in FIG. 6 includes two zones, an interior zone 202 and an exterior zone 204. The interior zone 202 is formed of a plurality of extremely high density, extremely high strength paperboard plies 202 a while the exterior zone 204 is formed from a plurality of high strength, but relatively compliant plies 204 a. In the core structure illustrated in FIG. 6, the zone of high strength but relatively compliant paperboard layers, 204, is advantageously positioned in the exterior 50% of the body wall of the paperboard core. Preferably, the zone 204 has a thickness 214, which constitutes about 30 to about 45% of the body wall thickness, most preferably between about 35 and 45% of the wall thickness, e.g., about 40% of the wall thickness. Similarly the interior zone 202, has a thickness 216, which constitutes about 50% to about 70% of the body wall thickness, most preferably between about 55% and 65% of the wall thickness, e.g., about 60% of the wall thickness.

[0049] When the high strength, relatively compliant density paperboard plies are positioned in a zone on or near the exterior of the body wall, like in the structure illustrated in FIG. 6, it is preferred that the total thickness 214 of the zone be at least about 5 mm, preferably from about 6 to about 9 mm. A preferred thickness 214 for the zone 204 of the 3 inch (76 mm) ID paperboard core structure illustrated in FIG. 6 is from about 6.8 to about 7.2 mm. The currently preferred total wall thickness of the (3 inch (76 mm) ID paperboard core illustrated in FIG. 6 is from about 17 to about 19 mm.

[0050] In addition, the core 200 of FIG. 6 can optionally include an outermost ply or plies 210, that are different from the extremely high density, extremely high strength paperboard plies of zone 202, and also different from the high strength but relatively compliant paperboard plies of zone 204. The outer ply or plies 210, when present, are varied in the manner and for the reasons set forth above in connection with the ply or plies 110 of FIG. 4, as will be apparent to those of skill in the art. Similarly, the interior-most ply or plies 211 of the core body 200 of FIG. 6 can be varied in the manner and for the reasons.

[0051] Yet another embodiment of the invention is illustrated in FIG. 7. The paper mill core of FIG. 7 is advantageously a 6 inch (150 mm or 152 mm) ID paper mill core. The core illustrated in FIG. 7 includes five zones, an interior zone 302, an exterior zone 312, a central zone 306, and two zones 304 and 308, positioned, respectively, between the interior and central zones, and between the exterior and central zones. In paper mill core structure of FIG. 7, the interior zone 302 is formed of a plurality of extremely high density, extremely high strength paperboard plies 302 a while the exterior zone 312 is also formed of a plurality of extremely high strength paperboard plies 312 a. Similarly, the central zone 306 is formed of a plurality of extremely high strength, extremely high density paperboard plies. The zones 304 and 308 are each formed of a plurality of lower density, relatively compliant high strength paperboard plies 304 a and 308 a, respectively.

[0052] In the paperboard core structure illustrated in FIG. 7, the interior and exterior extremely high strength, high density paperboard zones 302 and 312, respectively, each constitute about one-sixth of the total thickness of the wall structure. Likewise, each of the paperboard zones 304 and 308 formed of lower density, relatively compliant high strength paperboard plies, constitute about one-sixth of the total thickness of the paperboard body wall. The central zone 306, formed of extremely high strength, extremely high density paperboard plies preferably has a thickness of about one-third of the thickness of the body wall. The body wall preferably has a total thickness of at least about 15 mm. The core 300 of FIG. 7 can optionally include an outermost ply or plies 310, that are different from the extremely high density, extremely high strength paperboard plies of zones 302, 312, and 306, and also different from the high strength but relatively compliant paperboard plies of zones3O4 and 308. The outer ply or plies 310, when present, are varied in the manner and for the reasons set forth above in connection with the outer ply or plies of FIGS. 4 and 6, as will be apparent to those of skill in the art. Similarly, the interior-most ply or plies 311 of the core body 300 of FIG. 7 can be varied in the manner and for the reasons.

[0053] The paperboard core structure illustrated in FIG. 7 is currently not preferred; however, it does provide considerable strength properties while substantially decreasing distortion of the core exterior upon the application of significant radial force to the core interior.

[0054] The wide high strength paperboard cores of the invention in general have a length greater than about 100 inches, typically greater than about 120 inches (3 m) more typically about 142 inches (3.6 m) or greater. Advantageously, the paperboard cores have a minimal body wall thickness of about 13 mm and preferably have a total body wall thickness of at least about 15 mm or greater. Most preferably, the total body wall thickness will exceed the conventional body wall thickness used for a high strength paperboard core of corresponding ID. Thus, the body wall thickness of the 6 inch (152 mm) ID core illustrated in FIGS. 4 and 5 will preferably exceed about 15 mm., thus exceeding the standard 13 mm body wall thickness. Similarly, the body wall thickness of the 3 inch (76 mm.) ID core illustrated in FIG. 6 will preferably exceed the conventional 15 mm body wall thickness for 3 inch (76 mm.), ID cores.

[0055] The use of a total body wall thickness exceeding the body wall thickness of a conventional wide, high strength paperboard core of the same or comparable ID provides several significant benefits and advantages. In particular, increasing the body wall thickness compensates for the use of a smaller amount (i.e., a smaller number of plies) of extremely high density, extremely high strength paperboard plies, as compared to the conventional structures, while still providing a flat crush strength that is comparable to or exceeds the flat crush strength of the conventional structures. In general, the paperboard cores of the present invention will have an extremely high flat crush strength of at least about 200 lbs/in (3500 N/100 mm). For example, a conventional 6 inch ID high strength paperboard core formed entirely of extremely high density, extremely high strength paperboard plies has a flat crush strength of about 200 lbs/in (3500 N/100 mm). A preferred 6 inch ID paperboard core as illustrated in FIGS. 4 and 5 can readily have a total wall thickness of 16 mm (0.630 inches) and is thus thicker than the conventional structure in an amount of about 3 mm, thus representing a 23% increase in wall thickness. However, only about 65-70% of the total wall thickness in this case, is formed of extremely high strength, extremely high density paperboard; thus, the structure illustrated in FIGS. 4 and 5 is preferably formed from only about 80% of the extremely high strength, extremely high density paperboard as would be used to form the conventional structure. Nevertheless, the preferred paperboard core structure illustrated in FIGS. 4 and 5 can readily have a flat crush strength of about 220 lbs/in (3850 N/100 mm). In addition, the preferred structure illustrated in FIGS. 4 and 5 reduces the amount the core OD expansion for chuck engagement by about 30% compared to a conventional core.

[0056] Similarly, the preferred structures for 3 inch (76 mm.), inside diameter cores as illustrated in FIG. 6 can readily have a 25% core burst improvement (reduction in the amount the core OD expansion for chuck engagement) as compared to the conventional 15 mm wall thickness structure. Nevertheless, the flat crush strength of the preferred structures illustrated in FIG. 6 can readily be about 312 lbs/in (5425 N/100 mm) as compared to the conventional flat crush strength for a 15 mm body wall thickness 3 inch (76 mm.), ID core, of about 300 lbs/in (5250 N/100 mm). In addition, the dynamic strength properties of the paperboard core illustrated in FIG. 6 preferably is comparable to or exceeds the dynamic strength of the conventional comparable ID core.

[0057] The use of a higher wall thickness in accord with the present invention also enables attainment of a higher allowable rotational speed, or “critical speed”. Accordingly, the preferred paperboard cores according to the invention can be rotated at speeds from about 3 to about 5% greater than the critical rotation speed of conventional core structures. Preferably the increase in rotational speed is achieved by use of a total body wall thickness greater than about 15 mm in either embodiment of the invention The greater wall thickness associated with the preferred structures increases core OD. The increased OD results in a lower rotational speed for any web speed during winding and unwinding. Thus, the critical speed performance is improved.

[0058] Exemplary currently preferred paper mill winding cores according to the invention have the constructions set forth below. Construction 1 ID: 150.4 mm OD: 182.4 mm Wall Thickness (estimated): 16 mm Ply Density (g/cc) Thickness (micron) Inside* about 0.78 600 Plies 2-10 0.9  550 Plies 11-18 0.72 620 Plies 19-27 0.9  550 Ply 28** 0.68 740 Outer Ply NA 220

[0059] Construction 2 ID: 78.7 mm OD: 110.7 mm Wall Thickness (estimated): 17 mm Ply Density (g/cc) Thickness (micron) Inside* about 0.72 620 Plies 2-18 0.9  550 Plies 19-29 0.72 620 Ply 30** 0.68 740 Outer Ply NA 220

[0060] Construction 3 Wall Thickness (estimated): ID: 3 inch OD (estimated): 3.72 inch 0.717 inch Ply Density (g/cc) Thickness (inch) Inside* 0.76 0.025 Plies 2-21 0.82 0.022 Plies 22-32 0.68 0.025 Outer Ply** NA 0.013

[0061] Construction 4 Wall Thickness (estimated): ID: 6 inch OD (estimated): 6.63 inch 0.633 inch Ply Density (g/cc) Thickness (inch) Inside* 0.76 0.025 Plies 2-11 0.9  0.022 Plies 12-31 0.76 0.025 Plies 32-37 0.9  0.022 Ply 38** 0.76 0.025 Outer Ply** NA 0.013

[0062] Construction 5 Wall Thickness (estimated): ID: 6 inch OD (estimated): 6.63 inch 0.633 inch Ply Density (g/cc) Thickness (inch) Inside* 0.76 0.025 Plies 2-11 0.82 0.022 Plies 12-31 0.68 0.025 Plies 32-37 0.82 0.022 Ply 38** 0.76 0.025 Outer Ply** 0.65 0.013

[0063] Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation. 

That which is claimed:
 1. A multi-ply paperboard core for supporting a roll of wide, continuous paper mill sheet comprising: a multi-ply paperboard core structure having a length exceeding about 100 in. (255 cm.), and being defined by a generally cylindrical body wall having a thickness of at least about 15 mm; said body wall including a radially interior multi-ply zone constituting at least about 25% of the total thickness of the body wall and consisting essentially of paperboard plies, each having a density exceeding about 0.80 g/cc; and, a second multi-ply zone within the radially outer 70% of the total thickness of the body wall consisting essentially of paperboard plies each having a density of between about 0.65 and about 0.75 g/cc, said second zone having a thickness of at least about 4 mm.
 2. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 1 wherein said paperboard plies in said interior multi-ply zone each have a density between about 0.82 and 0.90 g/cc.
 3. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 1 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 4. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 2 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 5. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 1 wherein said interior multi-ply zone constitutes between about 30 and 35% of the total thickness of the body wall.
 6. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 5 wherein said second multi-ply zone has a thickness of between about 30 and 35% of the total thickness of the body wall.
 7. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 1 wherein said second multi-ply zone has a thickness of between about 40 and about 50% of the total thickness of the body wall.
 8. A multi-ply paperboard core for supporting a roll of wide, continuous paper mill sheet comprising: a multi-ply paperboard core structure having a length exceeding about 100 in. (255 cm.), and being defined by a generally cylindrical body wall having a thickness of at least about 15 mm, and an inside diameter of about 6 in. (152 mm); said body wall including a radially interior multi-ply zone constituting about 25% to about 35% of the total thickness of the body wall and consisting essentially of paperboard plies, each having a density exceeding about 0.80 g/cc; a second multi-ply zone within a central portion of the body wall consisting essentially of paperboard plies each having a density between about 0.65 and 0.75 g/cc., said second zone constituting between about 35% and about 45% of the total thickness of the body wall, said second zone having a thickness of at least about 4 mm.; and a third multi-ply zone within a radially outer portion of the body wall consisting essentially of paperboard plies each having a density exceeding about 0.80 g/cc., said third zone constituting between about 30% and about 35% of the total thickness of the body wall.
 9. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 8 wherein said paperboard plies in said radially interior multi-ply zone each have a density between about 0.82 and 0.90 g/cc.
 10. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 9 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 11. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 8 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 12. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 9 wherein said interior multi-ply zone constitutes about 33% of the total thickness of the body wall.
 13. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 9 wherein said second multi-ply zone has a thickness of about 33% of the total thickness of the body wall.
 14. A multi-ply paperboard core for supporting a roll of wide, continuous paper mill sheet comprising: a multi-ply paperboard core structure having a length exceeding about 100 in. (255 cm.), and being defined by a generally cylindrical body wall having a thickness of at least about 15 mm, and an inside diameter of about 3 in. (76.2 mm); said body wall including a radially interior multi-ply zone constituting at least about 50% of the total thickness of the body wall and consisting essentially of paperboard plies, each having a density exceeding about 0.80 g/cc; and a second multi-ply zone within the radially outer 50% of the body wall thickness consisting essentially of paperboard plies each having a density between about 0.65 and 0.75 g/cc., said second zone having a thickness of at least about 5 mm.
 15. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 14 wherein said paperboard plies in said radially interior multi-ply zone each have a density between about 0.82 and 0.90 g/cc.
 16. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 14 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 17. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 15 wherein said paperboard plies in said second multi-ply zone each have a density of between about 0.67 and 0.73 g/cc.
 18. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 15 wherein said interior multi-ply zone constitutes about 55 to about 65% of the total thickness of the body wall.
 19. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 18 wherein said second multi-ply zone has a thickness of between about 35 and about 45% of the total thickness of the body wall.
 20. The multi-ply paperboard core for supporting a roll of wide continuous paper mill sheet according to claim 19 wherein said body wall has a total thickness of about 16 mm or greater. 